Switching regulated power supplies operate off AC line voltage to convert the unregulated high voltage to regulated low voltage. In one common application, such power supplies are used to convert high AC line voltages to regulated low DC voltage, such as 5 volt, 12 volt, and 24 volt DC supply voltages employed for semiconductor integrated circuits in personal computer terminals. As shown in FIG. 1 of the accompanying drawings, the AC line voltage is full wave rectified to provide a high voltage unregulated DC bus voltage. This voltage is applied via a capacitor filter to a high frequency (typically, 20 to 500 kHz) DC to DC inverter circuit which performs pulse width modulation for output voltage control. Transformers are employed in the power supply to provide isolation between input and output and for voltage scaling.
The series of voltage pulses at the transformer secondary is applied to an output low pass filter circuit to provide an output voltage which is the average value of the input voltage, as follows: EQU Output voltage=(t.sub.on /T) (N.sub.2 /N.sub.1)E.sub.DC
where t.sub.on is pulse width; T is pulse cycle time; N.sub.1 and N.sub.2 are the number of windings on the primary and secondary, respectively, of the transformer; and E.sub.DC is the DC level of the AC input voltage to the power supply.
The output voltage of the supply is employed as the supply voltage for the semiconductor devices of the computer. The power supply efficiency is the ratio of power out to power in, and can be high--for example, greater than 70% for 5 volt outputs and greater than 85% for 24 volt outputs. The power factor, which is a measure of how well such a power supply utilizes the AC line voltage, however, is typically relatively low. A power factor of 0.7 is not unusual for supplies above 500 watts. Low power factor is attributable to the fact that the input current drawn by the rectifier and filter capacitor of the power supply is not sinusoidal and is not in phase with the input voltage.
As shown in FIG. 2A, superimposing current drawn by the supply on the line voltage, the current is drawn only in periodic pulses to recharge the input capacitor. A power factor improvement can be realized by increasing the conduction angle .phi., but this capability is limited by the ripple current rating of the input filter capacitor. For a typical conventional power supply with a .phi. of 1/4 of T/2 seconds, the demand is four times the RMS value of the input current.
As shown in FIG. 2B, voltage loss occurs as a result of high peak demand at the wall outlet for the power supply. The large peak current loading produces stress on facility wiring, and results in loss of peak AC voltage because of reactive and resistive regulation losses. It is not unusual to suffer as much as a 10% loss of peak AC voltage in a typical installation, with a concomitant deterioration of performance of other AC loads in use in the same facility.
A power supply having a power factor of 0.75 draws 25% more input current than a comparable power supply having a unity power factor. For example, a conventional 1000 watt, 5 volt power supply operating with a 0.75 power factor off a 115 volt AC input line will draw 16 amps, which exceeds the Underwriter Laboratories, Inc. limit of 15 amps per power outlet. In contrast, an otherwise identical power supply having a power factor of unity would draw only 12 amps under the same operating conditions, and would allow the AC voltage to remain sinusoidal rather than flattened because of supply loading as shown in FIG. 2B. The susceptibility of other loads to deterioration of performance is also reduced in the presence of a power supply operating with the higher power factor, at least partly because harmonic current is substantially reduced or virtually eliminated.
It is a principal object of the present invention to provide an improved power supply with high efficiency of power conversion.
It is another object of the invention to provide a regulated AC to DC converter having unity or near-unity power factor and relatively low input current demand.
Still another object is to provide a power supply which achieves high power conversion efficiency without adversely affecting the operating line range.
A more specific object of the present invention is to provide a powersaver circuit for use with or addition to a regulated AC to DC power supply, to improve the power factor, conversion efficiency, and operating line range impact of the supply.
Prior art correction circuits are typified by the disclosures in U.S. Pat. Nos. 2,367,625 to Short; 4,119,907 to Quinn; and 4,369,490 to Blum. Short describes a voltage regulator circuit including a capacitor, a linear reactor in series and a non-linear reactor in parallel with the capacitor, the volt-ampere characteristics of the reactors and the capacitor intersecting at a resonance point. Such a circuit provides voltage regulation but without improving the power factor. Blum discloses a low-ripple power rectifier system which includes a choke inductance and a capacitor, such a system being capable of power factor improvement but at the expense of reduced operating line range. Quinn describes a power factor corrector circuit having a filter which includes a fixed inductor and a capacitor, for a resistive load. The latter circuit also suffers from substantially reduced operating line range.